The quote in Act 2, Scene 2, lines 12-13, "Had he not resembled/my father as he slept, I had done't" indicates that Lady MacBeth does have some conscience.

The quote indicates that she has some knowledge of right and wrong because she shows compassion towards King Duncan by relating the appearance of the sleeping Duncan to memories of her father. The fact that she states she could not kill him as he slept there, helpless, resembling her father, indicates to the reader that she is not purely evil.

If you are searching for other instances of Lady MacBeth's humanity, consider this- it can be interpreted that guilt drove her mad. The idea that one could be so consmed with guilt that she hallucinates and eventually ends her life certainly shows knowledge that her actions were wrong. While I find this a compelling argument for her humanity, some may argue that Lady MacBeth's eventual insanity was a result of stress, pressure, or fascination, not guilt.

For more information about Lady MacBeth's character, I've linked to her character analysis.